Voltage detection

Abstract

A circuit arrangement for detecting an input voltage comprises an oscillator responsive to the voltage, the oscillator including a pair of transistors (8, 9) connected as an astable multivibrator, each of the transistors having a load which comprises a primary winding (16 or 17) of a current transformer (T) and a low resistance proof resistor (12 or 13). A current flows through a secondary winding (18) of the transformer the amplitude of which current is proportional to the input voltage.

I claim: 
     
       1. A circuit arrangement for detecting a voltage at an input and producing at an output a current dependent on the voltage, the arrangement comprising: an oscillator responsive to the voltage, the oscillator including at least one transistor having a load which comprises an impedance and a primary winding of a transformer, wherein said impedance is a low impedance proof impedance and said transformer is a current transformer, the current flowing through a secondary winding of the transformer and the output being isolated from the input by the transformer;   an operational amplifier being connected with the secondary winding of the current transformer, the amplifier having a first, high output level if the said voltage is of a first value, and a second, low output level if the said voltage is of a second, low value; and   opposite ends of said secondary winding being connected with a first input of the operational amplifier and a center tap of the secondary winding being connected with another input of the operational amplifier.   
     
     
       2. An arrangement according to claim 1, wherein the oscillator comprises first and second transistors connected as an astable multivibrator, the load of the first transistor comprising a first primary winding of the current transformer and a first low impedance proof impedance and the load of the second transistor comprising a second primary winding of the current transformer and a second low impedance proof impedance. 
     
     
       3. An arrangement according to claim 1, wherein the or each low impedance proof impedance is a low resistance proof resistor. 
     
     
       4. An arrangement according to claim 1, wherein a filter network including a resistor and a capacitor is provided for filtering an alternating current component in the said voltage, the said resistor of the network being a low resistance proof resistor. 
     
     
       5. A circuit arrangement according to claim 1, in an interlocking system for use in controlling a controlled element of a railway system.